Gas clathrate, also called gas hydrate or gas ice, is a solid form of water that contains a large amount of gas within its crystal structure. Such gas clathrates are found in formation fluids or natural gas, where hydrocarboneaous gas forms hydrates in conjunction with water. These hydrates usually exist in agglomerated solid forms that are essentially insoluble in the fluid itself.
Thermodynamic conditions favouring hydrocarboneaous gas hydrate formation are often found in pipelines, transfer lines or other conduits, valves and/or safety devices, vessels, heat exchangers etc. This is highly undesirable because the gas crystals might agglomerate and cause plugging or blockage of the flow-line, valves and instrumentation. This results in shutdown, loss of production, risk of explosion and injury or unintended release of hydrocarbons into the environment either on-land or off-shore. Accordingly, natural gas hydrates are of substantial interest as well as a concern to many industries, particularly the petroleum and natural gas industries.
Once formed, hydrates are difficult to decompose of, for instance by increasing the temperature and/or decreasing the pressure. WO-A-2007/055592 teaches a heat induction system for this purpose. However, at these conditions, the clathrate dissociation is a slow process. Therefore, preventing hydrate formation appears to be the key to the problem. Presently, hydrate formation may be often controlled by using chemicals, such as methanol, glycol, kinetic hydrate inhibitors or any other higher alcohol, and/or active heating. Remediation of a plugged conduit often employs some combination of active heating, chemicals and/or depressurization. The use of inhibition chemicals, depressurization and/or heaters may be logistically complex and expensive and may incur a certain amount of risk to field personnel. Above all, results are far from optimal.
US-A-2006/272805 suggests to impart energy to the mixture of gas and water, for instance using agitation or vibration. The hydrate particles continue to form as long as energy is imparted and water and hydrate guest molecules are available. High amplitude agitation of the gas and water will repeatedly break up agglomerated hydrate particles that form and encourage the formation of more and smaller particles. As more hydrate forms in this manner, less and less free water may be available proximate the gas and water contact. However, it is unclear how such an acoustic inhibitor would be enforceable in many of the applications where clathrates pose a problem.
Outside the field of gas clathrates, US-A-2005/065037 discloses coatings for well screens that protect the screens from damage as they are inserted into the wellbore and once in the well, release reactive materials to react with and degrade potentially plugging materials such as drill solids, fluid filtercakes, fluid loss additives, and drilling fluids. Apart from the fact that there is no hint in US-A-2005/065037 of the problem of clathrates, the polymeric coatings suggested therein for preventing solids deposition would even be of no use when dealing with the extreme problems of the clathrate “ice” crystals. One of the problems is that gas hydrates—once formed—form a new equilibrium with the water vapour phase, resulting in a lower water vapour pressure. As a consequence gas hydrates will further grow through deposition of the water vapour onto the hydrate surface.
It is thus an object of the invention to provide means for reducing or even avoiding the adhesion of gas hydrate to the interior surfaces of processing systems in hydrocarbons-containing gas streams that is not hindered by any of the aforementioned disadvantages, and can be readily applied to various conduits and parts thereof. It is also an object of the invention to prevent hydrate plugging in natural gas transport piping without requiring that the pressure integrity of the pipeline be compromised.